Van Noorden, R. September 24, 2014. Cheap solar cells tempt businesses. Nature #513 470-471.
[Excerpts. Of interest because rarely do obstacles get mentioned in the news. Most are optimistic hype making it sound like a solution to the energy crisis is just around the corner. And forget that electricity does not solve our main problem — heavy-duty trucks, locomotives, and ships run on diesel fuel — not electricity. Batteries for heavy-duty transport vehicles are so large the vehicle would barely move, and overhead lines are not practical over millions of acres of farmland, or other off-road logging-trucks, mining trucks, etc., nor could wires be strung over 4 million miles of roads, requiring trucks to have yet another power system after getting off the wires to get to their destination, which doubles the price of the truck].
Large, commercial silicon modules convert 17–25% of solar radiation into electricity, and much smaller perovskite cells have already reached a widely reproduced rate of 16–18% in the lab — occasionally spiking higher.
The cells, composed of perovskite film sandwiched between conducting layers, are still about the size of postage stamps. Seok says that he has achieved 12% efficiency with 10 small cells wired together.
Six reasons why perovskite cells might not be The Next Big Thing:
- To be practical, they must be scaled up, which causes efficiency to drop.
- Doubts remain over whether the materials can survive for years when exposed to conditions outside the lab, such as humidity, temperature fluctuations and ultraviolet light.
- Researchers have also reported that ions inside some perovskite structures might shift positions in response to cycles of light and dark, potentially degrading performance.
- The need for complex engineering might create another setback, says Arthur Nozik, a chemist at the University of Colorado Boulder. After plummeting in past years, the price of crystalline silicon modules — which make up 90% of the solar-cell market — has leveled off but is expected to keep falling slowly. As a result, most of the cost of today’s photovoltaic systems is not in the material itself, but in the protective glass and wiring, racking, cabling and engineering work.
- When all these costs are factored in, perovskites might save money only if they can overtake silicon in efficiency. In the short term, firms are focusing on depositing the films on silicon wafers, with the perovskites tuned to capture wavelengths of light that silicon does not. On 10 September, Oxford PV announced that it was working with companies to make prototypes of these ‘tandem’ cells by 2015, and that this could boost silicon solar cells’ efficiencies by one-fifth, so that they approach 30%. Malinkiewicz’s hope is to find a niche that silicon cannot fill: ultra-cheap, flexible solar cells that might not last for years, but could be rolled out on roof tiles, or used as a portable back-up power source.
- There is another potential snag: perovskites contain a small amount of toxic lead, in a form that would be soluble in any water leaching through the cells’ protection. Although Snaith and others have made films with tin instead, the efficiency of these cells is only just above 6%.